This invention relates to a comparator, a display apparatus which uses the comparator for a driving system and a driving method for the comparator, and more particularly to a comparator of the boosting type which outputs, as a comparison output signal, a signal of a higher level than that of a comparison input signal and a display apparatus as represented by a liquid crystal display apparatus or an EL (electroluminescence) display apparatus which uses the comparator as part of a circuit which forms a driving system as well as a driving method for the comparator.
As the process generation of the MOS LSI proceeds and the power supply voltage for a MOS LSI drops, also the output voltage of the LSI drops. As an example, if a peripheral circuit of a liquid crystal display apparatus is incorporated into an LSI, then the level of a driving pulse or display data supplied from the peripheral circuit in the form of an LSI to a driving system of the liquid crystal display apparatus becomes lower. However, in order to satisfy a demand for the picture quality of a liquid crystal display apparatus, it is necessary, under the present conditions, to apply a voltage of approximately 5 V to the liquid crystal.
Meanwhile, in a liquid crystal display apparatus which uses a TFT (thin film transistor) as a switching element for a pixel or the like, in order to apply 5 V to the liquid crystal, a driving voltage of approximately 10 V is required for a driver, a pixel transistor or the like formed from a TFT from characteristics of the TFT. In other words, it is necessary to mount a level shifter, a comparator or the like (conversion into an on-panel device) for converting display data of a small amplitude from the outside into display data of a TFT operation level on a liquid crystal panel (transparent insulating plate on which a pixel section is formed).
However, where a level shifter, a comparator or the like is formed as an on-panel device, there is a problem that characteristics of adjacent or neighboring transistors exhibit some dispersion, which is particularly remarkable with the TFT. Due to such dispersion, it is necessary to make the amplitude of an input signal greater than a certain level.
Meanwhile, reduction of power consumption is required particularly for a liquid crystal panel used for a portable apparatus. If an ordinary level shifter is mounted on a liquid crystal panel, then since it consumes much power, it is necessary also to take a countermeasure for reduction of power consumption. For the level shifter, for example, a comparator of the boosting type is used.
A comparator is a circuit which compares two or more input signals and outputs a comparison result of a high level (hereinafter referred to as xe2x80x9cHxe2x80x9d level) or a low level (hereinafter referred to as xe2x80x9cLxe2x80x9d level) in response to a relationship in magnitude among signal levels of the input signals. Usually, at least two transistors are used to compare input signal levels. The circuit system in this instance basically presupposes that characteristics of adjacent transistors such as the threshold voltage Vth, the drain-source current Ids and so forth are almost same. Actually, however, there is some difference between characteristics of adjacent transistors, and from this reason, the minimum comparison voltage is set to a comparatively high value.
FIG. 13 shows a comparator of the boosting type as a related art. Referring to FIG. 13, the comparator shown includes an N-channel MOS transistor Q101 and a P-channel MOS transistor Q102 connected in series between the ground and a power supply Vdd, and an N-channel MOS transistor Q103 and a P-channel MOS transistor Q104 connected in series between the ground and the power supply Vdd similarly. The gate electrode of the MOS transistor Q102 is connected to a drain common junction S between the MOS transistors Q103 and Q104, and the gate electrode of the MOS transistor Q104 is connected to a drain common junction T between the MOS transistors Q101 and Q102.
In the comparator described above, comparison input data Vdata is applied to the gate electrode of the MOS transistor Q101 through a switch SW101, and a comparison reference level Vref is applied to the gate electrode of the MOS transistor Q103 through another switch SW102. The switches S1WO1 and SW102 are driven to be switched between on (closed) and off (open) in response to the same timing pulse signal xcfx86s. A comparison output Vout is extracted from the drain common junction S of the MOS transistors Q103 and Q104. FIG. 14 illustrates a relationship in timing of the timing pulse signal xcfx86s, comparison input data Vin and comparison output data Vout.
In the comparator described above, if there is a difference in characteristic between the N-channel MOS transistor Q101 and the N-channel MOS transistor Q103, then when the level difference between the two input signals, that is, the comparison input data Vdata and the comparison reference level Vref, is small, a comparation error occurs. Further, since normally an intermediate value between the xe2x80x9cHxe2x80x9d level and the xe2x80x9cLxe2x80x9d level of the comparison input data Vdata is set as the comparison reference level Vref, through current flows through the MOS transistors Q101 and Q103. Accordingly, where a comparator of the boosting type is used for a driving system of a liquid crystal display apparatus, a number of such comparators equal to the number of bits of display data are provided for each of pixels in each horizontal row. Consequently, power is consumed as much.
As an example, where 2-bit data is used, if the number of pixels in each horizontal row is 100, then 200 (100xc3x972) comparators are required, and in a color system, since data for three colors of R (red), G (green) and B (blue) are required for each dot, totaling 600 comparators are required. In other words, as the number of bits of display data increases, also the number of comparators increases as much, and accordingly, the power consumption increases. It is to be noted that, if a single comparator substitutes them, since the single comparator drives a great capacity, the power consumption increases.
It is an object of the present invention to provide a comparator which can operate stably against an absolute value distribution of a threshold voltage among MOS transistors and has a wide allowable range against the threshold voltage dispersion and besides allows reduction in power consumption and a display apparatus which uses the comparator for a driving system.
In order to attain the object described above, according to an aspect of the present invention, there is provided a comparator, including a single MOS transistor, inputting means for inputting a comparison reference signal and a comparison input signal in a time series to a source electrode of the MOS transistor, current supplying means connected between a power supply and a drain electrode of the MOS transistor for supplying very small current to the drain electrode, voltage holding means connected between a gate electrode of the MOS transistor and a dc potential point, and switch means connected between the gate electrode and the drain electrode of the MOS transistor.
According to another aspect of the present invention, there is provided a display apparatus, comprising a driving system including the comparator described above.
According to a further aspect of the present invention, there is provided a driving method for a comparator which includes a single MOS transistor, current supplying means connected between a power supply and a drain electrode of the MOS transistor for supplying very small current to the drain electrode, voltage holding means connected between a gate electrode of the MOS transistor and a dc potential point, and switch means connected between the gate electrode and the drain electrode of the MOS transistor, the driving method including the steps of putting the switch means into an on state to short-circuit the gate electrode and the drain electrode of the MOS transistor to each other and simultaneously or thereafter applying a comparison reference signal to the source electrode of the MOS transistor, and putting the switch means into an off state so that a potential of the drain electrode of the MOS transistor is extracted as a comparison result and simultaneously or thereafter applying a comparison input signal to the source electrode of the MOS transistor.
In the comparator and the display apparatus, first within a preset period, a comparison reference signal is applied to the source electrode of the MOS transistor and the switch means is put into an on state. Consequently, a voltage with which a channel potential of the MOS transistor becomes substantially equal to the source potential (comparison reference signal) is held by the voltage holding means. Then, within a comparation period, a comparison input signal is applied to the source electrode of the MOS transistor and the switch means is put into an off state. Consequently, comparison of the source potential with the gate potential of the MOS transistor, that is, comparison of the comparison input signal with the held voltage (comparison reference signal) of the voltage holding means, is performed by the MOS transistor. Then, a result of the comparison appears at the drain electrode of the MOS transistor.
Thus, with the comparator and the display apparatus, since a single MOS transistor is used and a comparison input signal is compared in a time series with a comparison reference signal by the MOS transistor, a stable operation against a threshold voltage absolute value dispersion is possible, and reduction in power consumption is possible. Further, since comparation is performed not by means of two transistors, there is no influence of a relative dispersion (offset) of the threshold voltage and so forth in principle, and consequently, even if the signal level of the comparison input signal is low, a comparation operation can be executed without suffering from a comparation. Besides, since the circuit construction is simple, a high yield is obtained.
The above and other objects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings in which like parts or elements denoted by like reference symbols.